The aim of the present study was to evaluate the effect of metformin on body weight, body fat mass, waist circumference, lipid parameters, insulin secretion and insulin resistance in obese subjects with normal glucose tolerance. 26 subjects (16 females and 10 males), mean age 37.3+/-9.7 years and mean BMI 32.3+/-9.1kg/m2 were treated with individual hypocaloric diet and metformin at a mean dose of 2.38+/-0.32g for six months. Insulin secretion was studied during OGTT; insulin resistance was assessed by the HOMA index. We have found a reduction of 3.24% in body weight, which was accompanied by a decrease in body fat mass (by 7.45%, p<0.01) and in waist circumference (p<0.01). Insulin resistance decreased by 14.5% (p=0.02) after metformin. There was a significant decrease in the AUC for the total (by 17.6%, from 6999+/-1453 to 5763+/-1511mU/L.120min, p<0.01) and stimulated insulin secretion (by 17.1%, from 4215+/-1011 to 3495+/-925mU/L.120min, p=0.01) during OGTT after six-month treatment with metformin. In conclusion, the results from the present study demonstrate that metformin contributes to a reduction in body weight, body fat mass and waist circumference, improves insulin sensitivity and decreases basal, total and stimulated insulin secretion in obese subjects. Thus metformin appears to be an effective and well-tolerated drug in the treatment of obesity in subjects with normal glucose tolerance.